# deduction guides for `std::unordered_set`

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< cpp‎ | container‎ | unordered set

Defined in header `<unordered_set>` |  |
template< class InputIt, class Hash = std::hash<typename std::iterator_traits<InputIt>::value_type>, class Pred = std::equal_to<typename std::iterator_traits<InputIt>::value_type>, class Alloc = std::allocator<typename std::iterator_traits<InputIt>::value_type> > unordered_set( InputIt, InputIt, typename /* see below */::size_type = /* see below */, Hash = Hash(), Pred = Pred(), Alloc = Alloc() ) -> unordered_set<typename std::iterator_traits<InputIt>::value_type, Hash, Pred, Alloc>; | (1) | (since C++17)
template< class T, class Hash = std::hash<T>, class Pred = std::equal_to<T>, class Alloc = std::allocator<T> > unordered_set( std::initializer_list<T>, typename /* see below */::size_type = /* see below */, Hash = Hash(), Pred = Pred(), Alloc = Alloc() ) -> unordered_set<T, Hash, Pred, Alloc>; | (2) | (since C++17)
template< class InputIt, class Alloc > unordered_set( InputIt, InputIt, typename /* see below */::size_type, Alloc ) -> unordered_set<typename std::iterator_traits<InputIt>::value_type, std::hash<typename std::iterator_traits<InputIt>::value_type>, std::equal_to<typename std::iterator_traits<InputIt>::value_type>, Alloc>; | (3) | (since C++17)
template< class InputIt, class Hash, class Alloc > unordered_set( InputIt, InputIt, typename /* see below */::size_type, Hash, Alloc ) -> unordered_set<typename std::iterator_traits<InputIt>::value_type, Hash, std::equal_to<typename std::iterator_traits<InputIt>::value_type>, Alloc>; | (4) | (since C++17)
template< class T, class Alloc > unordered_set( std::initializer_list<T>, typename /* see below */::size_type, Alloc ) -> unordered_set<T, std::hash<T>, std::equal_to<T>, Alloc>; | (5) | (since C++17)
template< class T, class Hash, class Alloc > unordered_set( std::initializer_list<T>, typename /* see below */::size_type, Hash, Alloc ) -> unordered_set<T, Hash, std::equal_to<T>, Alloc>; | (6) | (since C++17)
template< ranges::input_range R, class Hash = std::hash<ranges::range_value_t<R>>, class Pred = std::equal_to<ranges::range_value_t<R>>, class Alloc = std::allocator<ranges::range_value_t<R>> > unordered_set( std::from_range_t, R&&, typename /* see below */::size_type = /* see below */, Hash = Hash(), Pred = Pred(), Alloc = Alloc() ) -> unordered_set<ranges::range_value_t<R>, Hash, Pred, Alloc>; | (7) | (since C++23)
template< ranges::input_range R, class Alloc > unordered_set( std::from_range_t, R&&, typename /* see below */::size_type, Alloc ) -> unordered_set<ranges::range_value_t<R>, hash<ranges::range_value_t<R>>, std::equal_to<ranges::range_value_t<R>>, Alloc>; | (8) | (since C++23)
template< ranges::input_range R, class Alloc > unordered_set( std::from_range_t, R&&, Alloc ) -> unordered_set<ranges::range_value_t<R>, hash<ranges::range_value_t<R>>, std::equal_to<ranges::range_value_t<R>>, Alloc>; | (9) | (since C++23)
template< ranges::input_range R, class Hash, class Alloc > unordered_set( std::from_range_t, R&&, typename /* see below */::size_type, Hash, Alloc ) -> unordered_set<ranges::range_value_t<R>, Hash, std::equal_to<ranges::range_value_t<R>>, Alloc>; | (10) | (since C++23)

1-6) These deduction guides are provided for `unordered_set` to allow deduction from an iterator range (overloads (1,3,4)) and std::initializer_list (overloads (2,5,6)). This overload participates in overload resolution only if `InputIt` satisfies LegacyInputIterator, `Alloc` satisfies Allocator, neither `Hash` nor `Pred` satisfy Allocator, `Hash` is not an integral type.
7-10) These deduction guides are provided for `unordered_set` to allow deduction from a `std::from_range_t` tag and an `input_range`.

Note: the extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified, except that as a minimum integral types do not qualify as input iterators. Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified, except that as a minimum the member type `Alloc::value_type` must exist and the expression std::declval<Alloc&>().allocate(std::size_t{}) must be well-formed when treated as an unevaluated operand.

The size_type parameter type in these guides refers to the size_type member type of the type deduced by the deduction guide.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_containers_ranges` | `202202L` | (C++23) | Ranges-aware construction and insertion; overloads (7-10)

### Example

Run this code

    #include <unordered_set>

    int main()
    {
        // guide #2 deduces std::unordered_set<int>
        std::unordered_set s = {1, 2, 3, 4};

        // guide #1 deduces std::unordered_set<int>
        std::unordered_set s2(s.begin(), s.end());
    }

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